The present invention generally relates to a detecting apparatus for detecting presence or absence of an item to be transported, and more particularly, to a detecting arrangement for detecting, for example, a copy paper wound around a photosensitive member in an electrophotographic copying apparatus. The detecting arrangement is adapted to optically effect the detection through employment of a sensor including light emitting and light receiving elements for detecting the item to be transported without contacting said item.
Generally, as a means for detecting presence or absence of items to be transported, there has conventionally been employed for a wide application, an arrangement which utilizes microswitches and the like. In such an arrangement, an actuating arm for controlling on or off of electric contacts in normally disposed on a transport passage of the item to be transported so as to detect the item when the actuating arm is moved based on the movement of said item. If the item to be transported is of a heavy item, the transportation thereof is seldom obstructed by the actuator arm disposed in the transport passage, and thus, an accurate detection may be expected.
However, if the item to be transported is of a very light weight item, for example, a piece of paper in the form of a sheet, there have been cases where the feeding thereof is obstructed by the actuating arm provided on the transport passage, thus resulting in paper jamming or the like. In the case where the paper sheet is rather thin, and is not sufficiently resilient, there is a stronger tendency for the jamming to take place. Accordingly, countermeasures have been taken, for example, by changing the shape of the actuating arm in various ways so as not to obstruct transportation of paper sheets by the actuating arm.
Meanwhile, there has been available a detecting arrangement which detects an item to be transported without any contact therewith. For the detecting arrangement of this type, an optical detecting means has been proposed for actual application, in which a light emitting element and a light receiving element are disposed, for example, with respect to a transport passage as a border line, so that light from the light emitting element is received by the light receiving element. There is also proposed an arrangement in which light emitting and light receiving elements are provided at the same face side for receiving light reflected from the item to be transported. In other words, detection of the item to be transported is effected through utilization of the fact that light is not received by the light receiving element when the light from the light emitting element is intercepted by said item to be transported. By the above arrangement, paper jamming and the like related to the detection may be advantageously prevented without any obstruction by the transportation of the item itself or regardless of the fact that the item to be transported is heavy or light in weight.
In the above case, a circuit arrangement, for example, as shown in FIG. 1 has conventionally been employed in general as a circuit for producing signals especially in the case of detecting presence or absence of paper sheets in a copying apparatus or the like.
In the circuit of FIG. 1, light from a light emitting element LED1 is arranged to be received by a light receiving element PT1 disposed through a transport passage. The light emitting element LED1 has its one end connected to a power source +Vcc through a resistor R1, and the other end thereof connected to a ground. Meanwhile, the light receiving element PT1 has the collector connected to the power source +Vcc, and the emitter to the ground through a resistor R2. While light is received by the light receiving element PT1, said element PT1 is conducted, and a voltage Vo at a point A of the emitter terminal becomes generally close to the power source voltage +Vcc, with the voltage Vo being further supplied to a (-) terminal of a comparator IC1 through a resistor R3. To a (+) terminal of the comparator IC1, there is applied a fixed voltage Vth ##EQU1## which serves as a reference voltage at a point B where the power source voltage +Vcc is divided by resistors R4 and R5. Accordingly, in the absence of any paper sheet between the light emitting element LED1 and the light receiving element PT1, the fixed voltage Vth becomes lower than the voltage at the point A, and therefore, the output of the comparator IC1 assumes "Low" state. Thus, when a paper sheet is transported between the light emitting element LED1 and the light receiving element PT1, said element PT1 is turned off, with the voltage Vo at the point A generally reaching the ground potential, and the output of the comparator IC1 is inverted from "Low" state to "High" state. Accordingly, presence or absence of the paper sheet is detected by the state of output of the comparator IC1, and the above signal is, for example, fed to a control section (not shown).
However, in the conventional circuit of FIG. 1 as described so far, when a comparison between the reference voltage Vth and the light detecting signal based on the light receiving element PT1 is to be effected, there have been cases where malfunctions take place due to increase of dark current in the light receiving element PT1 by heat, lowering of light emitting and light receiving amounts by the adhesion of paper scraps, dust and dirt, etc. onto the corresponding elements, and reduction of S/N ratio owing to deterioration with age and the like.
More specifically, for example, of the dark current for the light receiving element PT1 is increased by heat, even when light is intercepted by a paper sheet disposed between the both elements LED1 and PT1, the voltage Vo at the point A is raised by the dark current so as to be higher than the fixed voltage Vth, and the output of the comparator IC1 remains "Low" without being inverted to "High", thus resulting in an erroneous detection. Meanwhile, by the lowering of the light emitting and light receiving amounts due to adhesion of paper scraps, dust and dirt, etc. onto the light emitting element LED1 and light receiving element PT1 or adhesion of a developing material (particularly, toner) onto said elements in the case of an electrophotographic copying apparatus and the like, when the sensor output voltage Vo at the point A is reduced to be lower than the fixed voltage Vth, the output of the comparator IC1 becomes "High" to produce a signal indicating presence of a paper sheet, even if no paper sheet is transported, thus also resulting in an erroneous detection.
Moreover, in the known circuit of FIG. 1, it has been required to effect a fine sensitivity adjustment, since the detection for a presence or absence of the paper sheet is effected by setting the fixed reference voltage Vth. More specifically, owing to the fact that the characteristics of the light emitting element LED1 and the light receiving element PT1 tend to be largely scattered or deviated, variable resistors are employed for the resistors R1 to R5 in FIG. 1 so as to finely adjust the scattering in the characteristics of said light emitting and light receiving elements for effecting a proper detection.
Although the conventional circuit of FIG. 1 has been mainly described with reference to the arrangement in which the light emitting and light receiving elements are separately disposed via the transport passage to effect the detection through the item to be transported, the arrangement is not limited to the above, but the same circuit construction may be applied to the system in which the light emitting and light receiving elements are provided at the same face side for receiving the light reflected from the item to be transported.
The detection of paper sheets, etc. to be transported has been effected in the manner as described so far, but the paper sheet detection is not limited to be effected at the transport passage alone, but may also be effected, for example, with respect to a paper sheet undesirably wound onto a photosensitive member, i.e. a photoreceptor in the form of, for example, a drum or belt, etc. More specifically, in an electrophotographic copying apparatus and the like, a paper sheet is caused to closely adhere onto the photosensitive surface, for example, of a photoreceptor drum for transfer of a toner image formed on said photosensitive surface, onto the paper sheet. Therefore, after completion of the transfer, the paper sheet is separated from the photoreceptor drum so as to be further fed to a fixing section at a subsequent stage, but there are cases where the paper sheet undesirably remains closely adhering to the photosensitive surface of the photoreceptor drum without being separated therefrom, and is thus transported up to the portion of a cleaning device. If the paper sheet should be transported to the cleaning device as described above, the cleaning device itself or the photosensitive surface of the photoreceptor drum may be damaged in some cases, and therefore, it is necessary to detect the paper sheet wound onto the photoreceptor drum particularly before it reaches the cleaning device so as to stop the feeding of the photoreceptor drum. For the detecting means as referred to above, the mechanical switch detecting arrangement by the microswitches and the optical detecting means including the non-contacting light emitting and light receiving elements as described earlier are disposed to confront the photoreceptor drum.
In the mechanical detecting switch arrangement, either the actuating arm of the switch is directly detected by the photoreceptor drum or the paper sheet closely adhering to the photoreceptor drum is separated by a projecting item such as a sharp claw so as to move the actuating arm of the switch by the separated paper sheet. Therefore, if the photoreceptor drum has a soft surface of OPC (Organic Photoconductor) or PET (Polyethylene-terephthalate), there is a great possibility that the surface is undesirably damaged by the actuating arm or projecting item referred to above. Moreover, there has also been such an inconvenience that the copied images are soiled by the developing material particularly toner, etc. adhering to the actuating arm, and further, adhering to the photosensitive surface from the soiled actuating arm.
Concerning the above, when the sensor as described earlier with reference to FIG. 1 is employed, the drawbacks referred to above may be eliminated. However, the sensor arrangement also has the disadvantage that erroneous detection may result due to insufficient light emitting and light receiving amounts, when the light emitting and light receiving elements are soiled by the developing material, etc. Moreover, due to the disposition around the photosensitive member, there is a tendency that the light receiving element PT1 is particularly heated, with a consequent increase of the dark current, thus giving rise to frequent erroneous detections. For the prevention of such an inconvenience, it is necessary to increase S/N ratio of the sensor output for improvement, but there is a limitation to the increase of S/N ratio.
Furthermore, when the photosensitive member is in an endless configuration, the difference in the output signals between the photosensitive member and paper sheet may be detected, but if the photosensitive member has end portions, grippers, etc. provided at the forward end portions of the photosensitive member to hold said member on a drum or the like are output in the form of the same detection signal as for the paper sheet, thereby also giving rise to a faulty detection.